2.1 Field of the Invention
The invention relates to irradiated ethylene polymers and articles made using said polymers. More specifically, the invention relates to ethylene polymers that are modified by ionizing radiation, in air, at a temperature less than the crystalline melt temperature of the polymer, prior to article formation, as well as articles made from said polymers.
2.2 Related Art
Polyethylene is one of the most widely used commercial synthetic thermoplastic polymers. Polyethylene is inexpensive, easy to process and convert into articles, tough and flexible. In addition, polyethylene is a good electrical insulator over many frequencies. Polyethylene also exhibits better chemical resistance, including moisture stability, than other commercial plastics including nylon. Furthermore, thin films of certain grades of polyethylene are transparent. Polyethylene also exhibits excellent physiological compatability characteristics.
The commercial importance of polyethylene is enhanced by the wide variety of grades that exist, each having different physical properties and, therefore, different end-use applications. Polyethylene can be coated onto, or formed into, a wide variety of products using conventional conversion technologies such as extrusion, injection molding, cast molding, film and bottle blowing and powder coating.
Despite its popularity, polyethylene has several shortcomings. For example, polyethylene has relatively low mechanical properties (e.g., tensile strength, impact strength, flexural modulus, hydrostatic stress strength, etc.), poor environmental stress crack resistance (ESCR), low service temperature, and relatively poor adhesion to polar surfaces.
Crosslinking polyethylene is well known. For example, it is known to crosslink polyethylene with chemicals, e.g., peroxides, silane and/or multi-functional (meth) acrylates. Alternatively, it is known to crosslink polyethylene with ionizing radiation, e.g., using electron beam or gamma radiation.
Irradiating artifacts formed from polyethylene to improve mechanical properties and thermal stability is known. Such artifacts include polyethylene coated cables and wires, underfloor hot water piping, heat shrinkable polyethylene films, and polyethylene foams, gaskets and o-rings. Generally, a high degree of crosslinking (e.g., 60–70% gel content) is imparted by such processes.
However, there has been little research and development on, and much less commercialization of, polyethylene resins crosslinked by ionizing radiation prior to artifact formation. One reason for this lack of research is the understanding that high levels of crosslinking drastically decreases melt flow, making the polymer difficult to process and convert into artifacts. To the extent that irradiated ethylene polymers are discussed in the prior art, the irradiation of the polymer is generally done under non-ambient atmosphere and/or above the crystalline melt temperature and steps are generally taken to insure an extremely low gel content (e.g., below 0.5% by weight).
For example, U.S. Pat. No. 4,586,995 (“the '995 patent”) discloses “a novel polymer and polymer treatment method . . . [which] involves irradiation of a polymer under non-gelling conditions at a temperature above the polymer melting temperature and in the absence of oxygen.” See the '995 patent, abstract (emphasis added). The '995 patent, therefore, teaches away from processes that irradiate polymers under gelling conditions and/or at temperatures below the crystalline melt temperature and/or under unmodified atmosphere.
As further example, U.S. Pat. No. 6,114,486 (“the 486 patent”) relates to “a rheology-modified ethylene polymer having less than 0.5 weight percent gel . . . which is characterized as having improved rheological performance and/or melt strength attributes relative to the unmodified polymer.” See the '486 patent abstract (emphasis added). “The rheology . . . is modified . . . by treating the ethylene polymer in the presence of a crosslinking agent in an amount . . . less than the amount which would cause more than 0.5 wt % gel formation . . . Crosslinking agents include peroxide compounds, and other known heat-activated curing agents, such as azo compounds, and electron beam, gamma-ray, and other known radiation cure systems.” Id. at col. 13 lines 10–20. When radiation is used, “[t]he amount of energy used to modify the polymer is preferably at least 0.5 Mrad [5 kGy] . . . and typically up to 50 Mrads [500 kGy].” Id. at col. 14 lines 9–12. “The irradiation intensity is . . . adjusted to avoid substantial heating of the polymer, because that might cause the polymer to react with oxygen . . . unless additional measures are taken to prevent contact with oxygen.” Id. at col. 14 lines 18–25.The '486 patent, therefore, fails to recognize any benefit of crosslinking polymers with radiation versus chemical crosslinking, much less the benefits achieved using the irradiation conditions set forth herein, and also teaches away from the modification of polymers to contain any significant amount of gel or any significant reaction with oxygen.
In addition, European Patent Application No. 0047625 (“EPA '47625”) describes a process whereby “[a] blend of polyethylene having a density in the range of 0.935–0.948 g/cm3 and 1–3%, by weight, of carbon black and which has been irradiated in an inert atmosphere while in the form of granules, is fed to an extruder, extruded in the form of pipe and cooled.” See EPA '47625 abstract (emphasis added). “In a preferred embodiment the granules of polyethylene have been subjected to irradiation in an amount in the range of 0.05–0.75 Mrad [0.5–7.5 kGy] of irradiation and particularly 0.3–0.7 [3–7 kGy] of irradiation.” Id. at 4 lines 27–30. “Gel formation, on irradiation, in amounts of more than 0.1% by weight, is outside the scope of this invention in the unlikely event that it should occur.” Id. at 5lines 5–8. EPA '47625, therefore, teaches away from the irradiation of polyethylene under ambient atmosphere, teaches away from the irradiation of polyethylene in the absence of carbon black, teaches away from the generation of polyethylene with a gel content greater than 0.1% by weight and, finally, fails to recognize the benefits achieved using the irradiation conditions set forth herein.
Similarly, U.S. Pat. No. 3,130,139 (the '139 patent”) describes a method whereby “a uniform dispersion of carbon black in polyolefins is obtained by subjecting the polymer to ionizing radiation prior to its admixture with the carbon black.” See the '139 patent col. 1 lines 41–45. “As little as 0.0005 megarad [0.005 kGy] . . . is effective to improve the uniformity of dispersion of carbon black in polyolefins. As much as 10 megarads [100 kGy] or more of ionizing radiations can be used for purposes of the present invention.” Id. at col. 2 lines 10–15. The '139 patent fails to recognize the benefits obtained from irradiating polyethylene under the conditions set forth herein and, instead, utilizes a broad range of doses to further the dispersion of a single filler, carbon black, that is neither required nor preferred herein.
Finally, Great Britain Pat. No. 831,896 (“GB '896”) “[r]elates to the production of heat treated irradiated polymeric materials and blends of such materials with fillers.” See GB '896 p. 1 lines 10–12. The method comprises “subjecting a polymer of at least one 1-olefin . . . to a dosage of from 105 to 1011 roentgens [roughly 1 kGy to 1,000,000 kGy] of radiation, and thereafter heating the irradiated polymer to a temperature above its softening point.” Id. at p. 1 lines 49–59. “[T]he resulting product exhibits an increase in elongation and a decrease in stiffness, density and tensile strength over that of either the un-irradiated or irradiated starting polymer.” Id. at p.1 lines 19–24 (emphasis added). In the sole example, “test specimens were placed in aluminum cans which were swept out with nitrogen and then irradiated . . . .” Id. at p. 3 lines 112–115. GB '896 teaches away from the irradiation of polyethylene under ambient atmosphere and teaches away from benefits, such as increased tensile strength, obtained using the irradiation conditions described herein.
The inventors listed on this application have received prior patents relating to polymer irradiation including the following: South African Pat. No. 89/6852 (“S.A. 89/6852”) entitled “Crosslinking of Polymeric Materials,” South African Pat. No. 92/6738 (“S.A. 92/6738”) entitled “Process For Coating A Substrate With A Film Of Thermoplastic Polymeric Material; and South African Pat. No. 98/9245 (“S.A. 98/9245”) entitled “Crosslinking Of Polymeric Materials.” In general, these patents are directed toward the use of multi-functional crosslinking agents in conjunction with irradiation. Furthermore, these patents fail to recognize the revolutionary impact of carefully controlled irradiation on the properties of polyethylene feedstock.